1. Field of the Invention
The present invention relates generally to furnace rollers, and, in particular, to an improved furnace roller for conveying and supporting a heated workpiece.
2. Description of the Prior Art
In the field of strip steel production, furnaces can be roughly classified as being either of the batch or continuous type. Batch furnaces process an entire coil or slab at one time, while continuous furnaces feed the slab or strip through the heating zones on a continuous basis. These types of furnaces are heated tunnels containing internal furnace rollers to transport the heated workpiece, or strip of steel. For furnaces in the 1800xc2x0-2200xc2x0 F. range, cooling is generally required to keep the roll from becoming heat softened, over-oxidized, or subject to adhesive transfers with the conveyed product. For other furnaces that also operate at these higher heats, i.e., shuttle furnaces, water cooling is generally not available, so the rollers in these furnaces are subject to even more heat softening and other wear and tear issues. These failure modes are directly related to the roller temperature, and keeping the roller surface as cool as possible is desirable and increases the life of the roller. However, a cold roller is also a heat sink, and for most applications the resulting heat losses are unacceptable and create undesirable heat gradients. Therefore, a balance is needed between preserving the roller life through cooling, while, at the same time, conserving thermal energy by heating.
Typically, the hollow uncooled rollers use alloy outer shells, which operate at furnace temperatures. The alloy content of these rollers is such that they maintain sufficient strength and oxidation resistance at the required temperature.
A variant of this approach is evident in U.S. Pat. Nos. 5,702,338 and 5,338,280, which disclose the use of protrusions extending from the roller designed to take the brunt of the wear. These protrusions are commonly referred to as xe2x80x9ctiresxe2x80x9d and are axially spaced along the outer surface of the roller. In addition, these tires can be integrally cast with the support tube or added by fabrication. These tires are subject to a concentrated heated workpiece load, so they must be made of an alloy that is able to resist wear. However, due to their construction, the use of these tires increases the risk of damage to the underside of the heated workpiece or stab. Using the spaced, axially-located tires can create what is referred to as xe2x80x9cskid marksxe2x80x9d on the bottom side of the strip of steel. These skid marks and other irregularities caused by the use of tires give rise to serious quality issues.
Another class of rollers uses ceramic or mineral fiber materials as both the roller insulator and the heated workpiece contact surface. A water-cooled shaft supports the fiber construction. While this type of roller is thermally efficient and reasonably inexpensive, the refractory outer surface lacks wear resistance. This type of roll design is the traditional standard for stainless steel strip annealing furnaces. Again, the wear and tear of the ceramic or mineral fiber materials creates irregularities at the product contact surface. In addition, these ceramic or mineral fiber materials have life cycles that can be measured in weeks, as opposed to the life cycle of months for super alloys.
A third approach for furnace rollers uses a combination of insulation properties and alloy properties. For example, U.S. Pat. Nos. 5,341,568 and 5,230,618 disclose a water-cooled core which supports an array of larger diameter tires made of a suitable alloy. These tires operate at or near furnace temperatures and are responsible for product contact and conveyance. A refractory casing is built to a diameter slightly smaller than the tires, both inboard and outboard of the array. This refractory protects the water-cooled core tube from heat attack and also prevents thermal losses to the water system. Wear of the refractory is reduced because its outer surface is below the plane of contact of the tires. This roller design conserves furnace heat, while, at the same, conveying the product without excessive slab cooling. However, due to the differences in expansion and strength properties between the steel core and the refractory casing, repeated thermal cycles can lead to structural failure of the refractory, de-bonding of the refractory, and oxidative attack of the core tube, which eventually causes failure. Importantly, as discussed above, these tires can wear grooves and skid marks in the bottom surface of the slab, which can make it difficult to guide the slab into the next rolling operation. Further, these irregularities may cause quality and specification problems.
It is therefore an object of the present invention to provide a furnace roller that overcomes the deficiencies of the prior art. It is another object of the present invention to provide a furnace roller with a contact surface area with adequate strength and oxidation resistance at higher temperatures. It is still another object of the present invention to provide a furnace roller that does not subject the heated workpiece to adhesive transfers and skid marking on the bottom of the steel slab. It is yet another object of the present invention to provide a furnace roller that distributes the oxidative interactions with the entire slab area via a travelling helix, while limiting the amount of alloy material heat transfer at the roller counterface area.
Accordingly, we have invented a furnace roller for supporting a heated workpiece in a furnace. The furnace roller includes a roller body with a roller body outer surface. This roller body is configured to be rotatable along a roller longitudinal axis. The furnace roller also includes a tire or protrusion attached to the roller body outer surface. This tire is rotatable with the roller body and supports the heated workpiece, and the shape of the tire is such that a contact interface between the workpiece and the tire continuously shifts in a longitudinal direction.
The present invention also includes a method of conveying a heated workpiece in a furnace. In a first step, a plurality of substantially cylindrical tubular roller bodies are provided, each roller body having a roller body outer surface and configured to be rotatable along a roller body longitudinal axis. A tire is attached to the roller body outer surface and configured to be rotatable with the roller body, the tire supporting the heated workpiece. Next a flat-heated workpiece (a steel slab) is inserted into a furnace opening. Finally, the flat-heated workpiece is conveyed over the plurality of roller bodies, the tires supporting the heated workpiece during conveyance, such that a contact interface between the workpiece and the tire continuously shifts in a longitudinal direction.